Sulfonium salts polymerization initiators

There are also reports on the cationic polymerization of anhydrosugars having free hydroxy groups. Anhydrosugars shown in Scheme 27 were polymerized at high temperatures (>130 °C) with sulfonium salts as initiators to highly branched polysaccharides with in the range between 3 x lO and... 

Fig. 9. Initiation of epoxy cure. Irradiation of a triaryl sulfonium salt produces a radical cation that reacts with an organic substrate RH to produce a cation capable of releasing a proton. The proton initiates ring-opening polymerization. X = BF , PFg, AsFg, and SgFg. ...

Photopolymerization reactions are widely used for printing and photoresist appHcations (55). Spectral sensitization of cationic polymerization has utilized electron transfer from heteroaromatics, ketones, or dyes to initiators like iodonium or sulfonium salts (60). However, sensitized free-radical polymerization has been the main technology of choice (55). Spectral sensitizers over the wavelength region 300—700 nm are effective. AcryUc monomer polymerization, for example, is sensitized by xanthene, thiazine, acridine, cyanine, and merocyanine dyes. The required free-radical formation via these dyes may be achieved by hydrogen atom-transfer, electron-transfer, or exciplex formation with other initiator components of the photopolymer system. 

Previously, the same author [52] reported that compounds containing the tricoordinated sulfur cation, such as the triphenylsulfonium salt, worked as effective initiators in the free radical polymerization of MMA and styrene [52]. Because of the structural similarity of sulfonium salt and ylide, diphenyloxosulfonium bis-(me-thoxycarbonyl) methylide (POSY) (Scheme 28), which contains a tetracoordinated sulfur cation, was used as a photoinitiator by Kondo et al. [63] for the polymerization of MMA and styrene. The photopolymerization was carried out with a high-pressure mercury lamp the orders of reaction with respect to [POSY] and [MMA] were 0.5 and 1.0, respectively, as expected for radical polymerization. 

Cationic photoinitiators are compounds that, under the influence of UV or visible radiation, release an acid, which in turn catalyzes the desired polymerization process. Initially, diazonium salts were used, but they were replaced by more thermally stable iodonium and sulfonium salts. Examples of cationic initiators are in Table 4.3. 

Initiators based on halonium and sulfonium salts are used commercially in various microlithographic processes and in the coating industry. Onium salts were developed commercially as photoinitiators due to the lower sensitivity of cationic polymerizations to oxygen compared to radical polymerizations. Aromatic halonium and sulfonium salts with complex anions such as SbF6, AsF6 and BF4- do not initiate cationic polymerizations spontaneously, but must be activated by UV irradiation. 

Sulfonium Salts. Other onium salts beside the diazonium and halonium salts will, on exposure to light, initiate polymerization of epoxides and other monomers capable of undergoing cationic polymerization. The use of onium salts of the elements of Groups Va and Via was first described in 1975 (41, ) and 1976 ( ). 

The bulky anion then stabilizes the intermediate adduct from protonation of the epoxy group and then facilitates insertion of epoxide at the cationic propagation site. Rapid polymerization can then occur. Cationic photopolymerization of epoxides often involves the photo-generation of acid from an initiator such as diaryliodonium or triaryl sulfonium salts (Crivello, 1999). The anions are important in controlling the addition at the cationic site and are typically BF4 and PFg. The reactivity of the system depends also on the structure of the epoxide. 

Photoinitiated cationic polymerization has been the subject of numerous reviews. Cationic polymerization initiated by photolysis of diaryliodonium and triarylsulfonium salts was reviewed by Crivello [25] in 1984. The same author also reviewed cationic photopolymerization, including mechanisms, in 1984 [115]. Lohse et al. [116], reviewed the use of aryldiazonium, diphenyliodonium, and triarylsufonium salts as well as iron arene complexes as photoinitiators for cationic ring opening polymerization of epoxides. Yagci and Schnabel [117] reviewed mechanistic studies of the photoinitiation of cationic polymerization by diaryliodonium and triarylsulfonium salts in 1988. Use of diaryliodonium and sulfonium salts as the photoinitiators of cationic polymerization and depolymerization was again reviewed by Crivello [118] in 1989 and by Timpe [10b] in 1990. 

Recently, other onium salts such as /V-alkoxy pyridinium [13], allylic onium [14,15], trialkyl phenacyl ammonium [16], and dialkyl phenacyl sulfonium salts [17,18] of the following structures (Chart 11.2) are shown to be convenient for producing the initiating species for cationic polymerization. 

Protonic acids are efficient initiators for the polymerization of both sulfides and amines. The polymerization of thiiranes initiated with perchloric acid proceeds without induction periods. Induction periods are present, however, with methyl fluorosulfonate initiator 11). Secondary sulfonium salts are more reactive than tertiary ones (the opposite is true with oxonium ions)12) and induce rapid polymerization ... 

Unlike the photolysis of the triarylsulfonium ions that decompose irreversibly on irradiation, the photolysis of these new initiators is reversible. When irradiated, a photostationary state is established in these initiators, as indicated in Reactions (6.50) and (6.51). When the irradiation is stopped, the ilyd and the Brpnsted acid revert to the original sulfonium salt. The acidity of the polymerization medium has to be maintained for the propagation of the ionic chain. When the acid is exhausted, the growth of the polymer chain is inhibited and polymeriz-ation stops. 

Due to the high effectiveness of the iodonium salt developed, it is used preferentially in the following investigations as a photoinitiator for the polymerization of epoxides. In some cases a commercial sulfonium salt is also used, in spite of the known disadvantages. In other cases the polymerization is initiated thermally. 

Cationic polymerization of diethyleneglycol divinyl ether and butanediol divinyl ether in the presence of oniiim salts was induced by y-irradiation. The mechanism for the initiation process involves the reduction of onium salts either by organic free radicals or solvated electrons depending on the reduction potentials of the onium salts. The reduction potentials of sulfonium salts was determined by polarography at the dropping mercury electrode. Only solvated electrons were capable of reducing the salts with reduction potentials lower than approximately -100 kJ/mol. The redox process liberates the non-nucleophilic anion from the reduced onium salt and leads to the formation of a Bronsted acid or a stabilized carbenium ion. These species are the true initiators of cationic polymerization in this system. The y-induced decomposition of onium salts in 2-ethoxyethyl ether was also followed by measuring the formation of protons. An ESR study of the structure of the radicals formed in the y-radiolysis of butanediol divinyl ether showed that only a-ether radicals were formed. 

Crivello s pioneering work on onium salt-type photoinitiators (sulfonium and io-donium salts) gave great impetus to investigations of cationic polymerizations [5, 6]. A common feature of mechanisms proposed in relation to onium salt-type initiators of the general structure is the generation of... 

Higher conversions in thiirane polymerizations, however, proceed with chain scission transfer mechanism under the influence of BF3 (C2H5)20 [192]. This is indicated by a change in the molecular weight distribution, a bimodal character. When the reaction is complete there is a marked decrease in the average molecular weight of the polymer. When thietane polymerizes with triethyl-oxonium tetrafluoroborate initiation in methylene chloride, the reaction terminates after only limited conversion [193]. This results from reactions between the reactive chain ends (cyclic sulfonium salts) and the sulfur atoms on the polymer backbone. In propylene sulfide polymerization, however, terminations are mainly due to formations of 12-membered ring sulfonium salts from intramolecular reactions [193]. 

Over the past several years, there have been developed several new classes of onium salt photoinitiators capable of initiating cationic polymerization. The most significant of these are aryldiazonium salts, diaryliodonium salts, triarylsulfonium salts, and dialkylphenacyl-sulfonium salts. The mechanisms involved in the photolysis of these compounds have been elucidated and will be discussed. In general, on irradiation acidic species are generated which interact with the monomer to initiate polymerization. Using photosensitive onium salts, it is possible to carryout the polymerization of virtually all known cationically polymerizable monomers. A discussion of the various structurally related and experimental parameters will be presented and illustrated with several monomer systems. Lastly, some new developments which make possible the combined radical and cationic polymerization to generate interpenetrating networks will be described. 

This modifieation does extend the sensitivity to longer wave length ultraviolet light. The eompounds were reported to display exeellent thermal lateney in the presence of various monomer systems and high effieieney as photoinitiators for cationic polymerization. Furthermore, their initiation efficiency was reported to be on par with current commercial triaryl sulfonium salts. 

Other Onium Salts and Organometallic Photoinitiators. The success of the iodonium and sulfonium salts as photoinitiators has led to the investigation of a number of analogous onium salts based on the halides and the Group VIA atoms however, these alternative initiators have not been widely used for various reasons. For example, chloronium and bromonium salts were prepared (57,58) and they were also found to function as cationic photoinitiators, but these salts are difficult to prepare and they have low thermal stability. Similarly, triarylselenon-ium salts have also been investigated and foimd to function as cationic initiators (59) however their preparation has been foimd to be expensive (60). Other onium salts such as phosphonium and arsonium salts, developed by Abu-Abdoun and co-workers for the photopolymerization ofp-methylstyrene and styrene, have also been reported (61-63) as successful cationic photoinitiators. Photopolymerization of carbazolyloxiranes with sulfonium and tropylium salts has been reported (64). Dialkylphenacyl sulfonium photoinitiator (65-67) has been reported with excellent solubility in both polar and nonpolar monomers. Pyridinium and isoquino-linium salts have also been reported and they were found useful for polymerizing both the epoxide and vinyl ether monomers (68). 

Aryldiazonium salts as UV initiators were described as early as 1973 [14]. Cationic polymerization with these initiators is not oxygen sensitive. This first class of salts had poor pot life and had problems due to liberation. The next generations, diaryl iodonium and diaryl sulfonium salts, overcame most of these problems. They were developed about the same time by 3M and General Electric [15-18]. Cationic systems utilizing cycloaliphatic epoxies exhibit the interesting feature that once polymerization begins, it can continue in the dark. Compared to free radical methods, use of cationic UV cure remains relatively small, but has found certain niches, such as metal varnishes. 

It was not until the invention of iodonium and sulfonium salts as photo-initiators by Crivello (1975) that cationic photo-polymerization became practical (see Crivello et al., 1977,1990,2000). Upon irradiation of these Crivello salts, acids are generated. Another significant difference between free radical and cationic polymerizations is the latter process is a living polymerization— once the acid species is formed, it remains active even after the irradiation is stopped. In contrast to this behavior free radicals die soon after irradiation is stopped. Also, unhke free radical polymerizations cationic reactions are not inhibited by oxygen. Quite often the dark reaction following irradiation can play an important role in enabhng a cationic system to develop its full properties and this leads manufacturers of commercial cationic photopolymers to often recommend a thermal (dark) postcure after carrying out the photo-irradiation process. 

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